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RE: [RPRWG] Question about the Fairness Message

All fairness messages are are required to be sent with a TTL of 255.
The receiving station can compute the number of hops to the node 
that originated the message using (256 - TTL in the packet).  

Type B messages are broadcast so every station will receive N of these.
However these are not used by the MAC.  The information from Type B
messages is used by the MAC client.

Type A messages are sent hop by hop.   Every station receives only
one of these.  Generation of Type A's is as follows.

When a node is congested, if it is more congested than the 
node from which it received the Type A message (i.e. its fair rate is less 
than what is in the received message), then the local node advertises its 
fair rate (with its SA and with a TTL of 255).  Otherwise, the downstream 
node's value is passed without modifying the SA, and the existing TTL 
in the frame is decremented.  

When a node is not congested,  if forward_rate_congestion is greater 
than allow_rate_congestion, the downstream node's value is passed 
without modifying the SA and the TTL is decremented.  Otherwise, a NULL 
rate is sent with the local node's SA and with a TTL of 255.  

In this way, every node only knows about the most congested node that 
it sends traffic to.  It can send unlimited traffic up to that congested
It must limit its transmit rate to the fair rate advertised by the congested
node for traffic destined beyond the congested node. 

Hope this helps.

> -----Original Message-----
> From: Zhou [mailto:xbzhou@xxxxxxx]
> Sent: Thursday, May 30, 2002 12:47 AM
> To: stds-802-17@xxxxxxxx
> Subject: [RPRWG] Question about the Fairness Message 
> Hi,
>   As defined in D0.2, the type A and type B fairness messages 
> are sent out periodically to propagate fair rate information 
> to upstream nodes. In another word, it means that all 
> congested or uncongested nodes both send this fairness 
> message onto the ring. Assume that one node received the 
> fairness message from another node, but how could the node 
> judge which node is congested ? And then how could the node 
> get the number of hops to the congested node? 
> Best Regards,
> X.B.Zhou
> PHD. of Tsinghua University
> email:xbzhou@xxxxxxx
> 2002-05-30